I was seated near the rear of a DL md-90 and saw this yellow spiral as I looked back at the engine. Probably so they can tell how fast the fan is spinning by looking at the spiral, or to scare birds away.

JAL did a study with its 747s having some airplanes with engine spinners that were painted black and some airplanes with spinners painted with the spiral pattern. The different airplanes were operated on similar routes. JAL's study clearly showed that the airplanes with spiral-painted spinners had fewer birdstrikes. The irony is that despite its test results, JAL did not paint its remaining spinners.